The expression $62.4d-210$ gives the number of Indian rupees you receive when you exchange $\$d$ at the local currency exchange. How many Indian rupees do you receive when you exchange $\$13$ ?
Answer: Exchanging $\$13$ tells us that $d={13}$. Let's substitute $d={13}$ into the expression and evaluate: $\begin{aligned} &\phantom{=}62.4d-210\\\\ &= 62.4\left({13}\right)-210\\\\ &= 811.2-210\\\\ &= {601.2}\\\\ \end{aligned}$ You receive ${601.2}$ Indian rupees when you exchange ${\$13}$.